For the production of components of fiber-reinforced plastic materials, various methods are known that are, in part highly, different in terms of their properties and the required technology.
Depending on the intended application, it is always necessary to find a compromise between the desired lightweight construction, the properties to be achieved, and economic efficiency.
This is because, on the one hand, the high-strength, high-modulus fibers employed, for example, the carbon (C) fibers or glass fibers (GF), are very cost-intensive; on the other hand, the methods such as prepreg technology, hand lay-up method or autoclave manufacturing are very time-consuming and difficult to automate because of a high degree of manual production, and lead to expensive semi-finished products.
These methods have the disadvantage that textile sheets, woven fabrics, mats, braids or knitted fabrics have to be prepared first in a complicated process, and then have to be soaked with resin for the prepreg method, are cut and laid. On the other hand, relatively large waste cuttings are obtained. A complicated storage is necessary in order to avoid curing and depolymerization.
The further processing may take place in hot-press technology or autoclave technology.
Another solution for producing oriented fiber composite components is the winding technique, in which the material is wound around a mandrel. Such a method is known, for example, from DE 10 2004 003 749 A1. A drawback of the winding technique is the limitation to components with convex surfaces. DE 10 2011 078 709 A1 describes a method in which the shape of the core can be changed after winding.
For large-scale production, the conflict between lightweight construction with carbon fibers on the one hand and the cost for the use of CFRPs on the other hand is particularly clear. In addition to the material price, it is mainly the time-intensive technologies that make series production difficult.
For the automotive industry, for example, it is essentially the production cost that is to be lowered according to Prof. Ferkel [Prof. Dr. H. Ferkel, CCeV Automotive Forum 2010, Jun. 24, 2010, Fiber Composite Materials as an Economic Option for Large-Scale Production].
Methods with a high fiber orientation, which allow for a higher automation, are limited to simple components (winding, pultrusion methods).
Methods suitable for series production, such as injection molding, SMC, GMT and LFT, make use of the fibers' potential only partially.
The properties of the components are determined, on the one hand, by the kind of fibers, the fiber arrangement, the fiber proportion, the fiber length and the fiber orientation and, on the other hand, by the matrix material and the interaction of the fibers with the matrix material.
DE 2 145 442 A and DE 2 300 269 A, which is based on the former, describe a preshaped part of fiber material as well as a method and a device for the production thereof. In particular, a preshaped part is created that can be used for the production of fiber-reinforced objects in a mat shape, such as kettles, tanks or the like; in this case, the fibers of the preshaped part are to be distributed uniformly along the entire wall surface. The device for performing the method is a perforated mold that is rotated around its longitudinal axis, and to the interior of which a vacuum is applied. Two chopping means cut the reinforcing fiber material, for example, glass fibers, into short fiber pieces, and direct the fibers onto the rotating mold. The vacuum applied to the perforated mold causes an arbitrary orientation of the fibers. Since the mold rotates with respect to the chopping tool while the latter is moved along the mold to apply the fibers thereto, the predominant orientation of the fibers runs helically around the perforated mold. Thus, the method is not suitable for producing geometric “three-dimensional” objects, because a high proportion of cuttings would be discarded. In addition, the method is not suitable for producing objects with partially different weights per unit area, either.
EP 0 587 283 A1 describes a fiber preform in which a binder and chopped fibers are blown from corresponding nozzles onto a screen. At the same time, heated air is blown from a plurality of further nozzles onto the forming fiber preform to cure the binder. WO 93/20994 A describes a method for producing a fiber preform in which a powdery binder on chopped glass fibers that are on a preforming screen is described. The powdery binder and the chopped glass fibers are then heated to soften the particles of the binder, whereby the latter adheres to the glass fibers to form the preform.
WO 2005 030 462 A2 describes an apparatus and a method for producing fiber preforms in which fibers and binder have been disperses hot on a surface such that the materials are conditioned and then solidified on the surface. The raw materials of the fiber preform contain reinforcing fibers, which are mixed with binder, especially thermoplastic or thermoset materials. Subsequently to the application to the surface, the fibers are further heated, cooled and solidified. This produces a fiber preform in the form of an open mat having interstices between the reinforcing fibers. The deposited mat can also be shaped further into a final desired shape before complete solidification.
DE 10 2011 120 986 A1 describes an injection molding process for manufacturing a fiber composite component in which a core of the component is cast at first in a first mold. Then, fiber material is soaked with plastic, and the soaked fiber material is wound around the cast core. Subsequently, heat is applied to the core with the wound material in a second injection mold, whereby the fiber material is fixed. Then, the core-filled hollow profile is demolded, and the core is molten out of the hollow profile by heating.
EP 2 543 494 A2 also describes a process for producing a component of fiber-reinforced plastic. At first, a core is provided in a fiber application device. Then, fibers impregnated with a plastic matrix are applied to the core. The plastic matrix is cured in one process step by applying pressure and/or heat to the fibers. According to EP 2 543 494 A2, the core can be provided with a surface comprising a shape-variable portion.
In the production of such conventional three-dimensional components, it may occur that cavities are formed perpendicularly to the direction of the fibers, caused by drawing uniaxial fibers perpendicularly to the direction of the fibers. Such cavities are then filled by the plastic material. These regions then have a significantly lower strength than that of the reinforced regions, leading to worse material properties.
In contrast, it is known that non-wovens have the best deformation behavior, i.e., they are best suitable for being adapted to a desired contour. But also in non-wovens, the highest dilution of material takes place during the shaping in the regions with the highest deformation. This means that strength properties of the component are determined by the geometry of the component even if a conventionally produced non-woven is used.
In addition, in the production of components made of non-woven with a stronger deformation, it may occur that the non-woven is shifted from the periphery into more interior areas when the mold is being closed.
Therefore, a significant improvement is offered by fiber preforms in which the fibers employed are directly placed into a three-dimensional mold, and only then are solidified.
For insulations in the floor and bulkhead regions of vehicles, a wide variety of fiber injection methods for producing fiber non-woven parts are known that avoid many of the mentioned problems.
EP 0 909 619 B1 describes an invention for the discontinuous manufacture of shaped composite materials. In this method, a mixture of fibers is sucked onto a perforated drum having the shape of the component. The thickness of the component is defined by skim rollers. In a final step, the component is solidified by heat. In this method, a problem arises in the adjustment of different weights per unit area for the defined adjustment of the local flow resistance.
EP 2 305 869 B1 (WO 2014/053505 A1) describes a process in which mixtures of fibers are blown through one or more blowing apertures into a mold. Subsequently, the material is solidified by heat. For stiff fibers, such as carbon or glass fibers, this method represents a possible cause of fiber breaking during the solidification because of the more or less non-two-dimensional arrangement of the fibers.
EP 0 704 287 B1 is a method for producing fiber mixtures in which a semifinished product is prepared by an air layering method. In a second step, this semifinished product is pressed as a hot component in a second mold, and cooled.
WO 2007/016879 A1 describes a combined blow-forming method in which the mold has two broad side walls, at least one side wall being contoured. The fiber mixture is blown into this mold, and sucked off sector by sector. The fiber blank is withdrawn by a take-off device and solidified. In this case too, the not solely two-dimensional arrangement of the fibers is a drawback for carbon and glass fibers.
DE 10 2005 004 454 B3 describes a method for the preparation of textile molded parts. In this method, fibers with a dissimilar layer thickness are collected to form a blank and sucked into a mold that has the shape of the blank and air-permeable walls.
EP 2 108 497 B1 describes a method for producing component parts in which a fiber mixture of fibers and binding fibers is blown/sucked against gravity into an air-permeable flock tool with the one-sided contour of the component part.
In all of these methods, released fibers are conveyed by sucking and/or blowing to the desired place in an air-permeable tool. For this application, the fibers employed comprise from 60% by weight to 85% by weight higher-melting one-component fibers, and from 15% by weight to 40% by weight of binding fibers.
According to the invention, mainly bico fibers (bicomponent fibers, for example, with a high-melting core and a low-melting shell) based on PET as the core and CoPET as the shell are employed as binders. Other shell materials on the basis of PE or PP are also known.
The material is partially or completely bonded by heating with air. If the non-woven component is only partially bonded, post-solidification is performed in a heated tool.
For components made of carbon fibers, such methods for producing fiber preforms are neither known nor employed. Since the fiber preforms have a low solidification, the methods for bonding the fibers into a plastic matrix can be applied only conditionally.